Thin film deposition or removal requires either chemical or physical processes or a combination of the two and most frequently takes place under conditions off a partial vacuum. A typical film removal system to which the equipment and method of the current invention could be conveniently applied is depicted in FIG. 1.
The method of film removal depicted here is commonly referred to as dry etching or reverse sputter etching depending on the pressure level maintained during the process. The substrate 20 is placed on an electrode 21 which may be electrically isolated or part of the electrical ground of the system. A second electrode 22 is connected to the opposite polarity of a power supply unit 25. Commonly this is the positive polarity. The system is enclosed within a vessel 23 which is evacuated by a pumping means 24. The application of power from the power supply 25 ionises residual gas in a the vessel or alternatively additional gases may be introduced in order to modify the environment and the process. The ionized gases are attracted to the electrodes with the heavy positively charged ions impinging on the substrate 20 causing film removal by physical means and/or chemical means.
It will be readily observed from the foregoing description and the drawing that the introduction of any probe into the etch region will prevent ions from impinging on the whole substrate and, if the probe is metallic, disturb the electrical profile within the etch region to the detriment of the process. As such it is common and well known to introduce an optical signal which is reflected off the substrate and subsequently detected. A typical optical path is shown at 28 with access to and egress from the system made possible by transparent food through ports or windows 26,27. An alternative system is to provide a small window in the electrode 22 so that light can be directed at the substrate and reflected back along its own path.
An alternative arrangement for deposition rather than removal of thin films is shown in FIG. 2.
The method of film deposition depicted here is commonly referred to as sputter deposition or plasma enhanced chemical vapour deposition depending on the pressure level maintained during the process. The substrate 30 is placed on an electrode 31 which may be electrically isolated or part of the electrical ground of the system. A second electrode 32 is connected to the opposite polarity of a power supply unit 35. Commonly this is the negative polarity. The system is enclosed within a vessel 33 which is evacuated by a pumping means 34. The application of power from the power a supply 35 ionises residual gas in the vessel or alternatively additional gases may be introduced in order to modify the environment and the process. The ionised gases are attracted to the electrodes with the heavy positively charged ions impinging on the chosen material to deposit 39 which is placed on or bonded to the electrode 32. Material is then deposited by physical or chemical or a combination of methods on the substrate 30. As a variant on this process there may be no deposition material 39, with the deposition occurring by a chemical combination of gases enhanced by the plasma.
As with the previous case, it will readily be seen that the introduction of a physical probe, such as may consist of a quartz crystal microbalance, into the deposition region will prevent depositing material from impinging on the whole substrate and, if the probe is metallic, disturb the electrical profile within the etch region to the detriment of the process. As such it is common and well known to introduce an optical signal which is reflected off the substrate and subsequently detected. A typical optical path is shown 38 with access to and egress from the system made possible by transparent feed through ports or windows 36,37. An alternative system is to provide a small window in the electrode 32 so that light can be directed at the substrate and reflected back along its own path. As an alternative if the substrate is transparent then a small hole can be introduced in the electrode 31 with light transmitted through the substrate.
Light that is introduced as described above reflects off the film that is being deposited or removed and the properties of the reflected light are modified (Ref Born and Wolf). Such modification will occur to the intensity of reflection and/or to the polarisation properties and these modifications will depend on the wavelength of the incoming optical radiation. Determination of the film thickness can be by reference to an existing reference standard (Ledger et al, EP 0 545 738 A2) or alternatively oscillations in reflected monochromatic light can be counted (Corlias, GB 2 257 507 A). These methods can be improved by the introduction of additional wavelengths (Canteloup et al, EP 0 735 565 A1) where the additional wavelengths, or indeed white light illurination with spectral analysis of the reflection, is used to remove anomalies in the identification of a particular oscillation extremum.
Prior art assumes an idealized development of the reflection process (FIG. 3) with the change in film thickness between extrema in the reflection signal (50) occurring in a time .DELTA.T being given by the relationship:
.DELTA.x=.lambda./(4.mu.) PA1 .lambda. is the wavelength of the light used to probe the film thickness; and PA1 .mu. is the refractive index of the material at the wavelength of light .lambda.. PA1 providing a means for reflecting or transmitting light through or from a thin film structure whilst that film structure is being processed to increase its thickness, decrease its thickness or otherwise change a property that relates directly or indirectly to its optical properties; PA1 at each point in time constructing an algorithm for processing the changing optical signal by direct reference to a set of calibration data, such set of calibration data either having been previously acquired from a calibration run of the process or, preferably, generated from a physical model of the thin film structure's development with thickness; the defining essential of the algorithm being that it is not sensitive merely to signal level but is highly sensitive to development of the signal wave-form shape with changing thickness; and PA1 providing a means for indication of rate of change of thickness (or other derived parameter) with time for indication and control together with a means for indication of thickness (or other derived parameter) with time for indication, control and cessation of the process.
where .DELTA.x is the change in film thickness occasioning the change in reflection level;
In real situations the signal frequently does not meet this ideal and resembles the signal obtained and illustrated in FIG. 4.
The structure of the film giving this reflected signal during its etch is shown in FIG. 5. Here a metallic mask 61 is overlying a film of silicon oxide 62 on a silicon substrate 63 and the illumination beam 64 is such that both the mask 61 and the exosed film are illuminated. The idealised reflection profile (which can be calculated as discussed below) is shown in FIG. 6. By comparing the idealised situation (FIG. 6) with the practically experienced situation (FIG. 4) a number of features are apparent.
Firstly there is the presence of wide bandwidth noise.
Secondly there is a variation in the actual signal variation between extrema (from maxima to minima).
Thirdly the fine detail structure in the trough at each minimum has been completely masked.
It is the prime objective of this current invention to provide a signal processing means to optimise the acquisition of information from the signal of the type shown in FIG. 4.